This invention relates to vacuum brazing of clad aluminum sheet components into an integral structure to manufacture an automotive evaporator core or the like. More particularly, this invention relates to a vacuum brazing cycle that includes a pre-brazing dwell at a temperature near the aluminum-silicon eutectic melting point to partially melt the clad alloy prior to heating to a higher temperature to complete melting and effectuate flow into joints
Automotive radiators and the like are formed by brazing an assembly of clad aluminum sheet components. The components are formed of a low-silicon aluminum alloy core having a cladding composed of a hypoeutectic aluminum-silicon braze alloy A typical aluminum braze alloy contains between about 9 and 11 weight percent silicon and is composed predominantly of eutectic phase, but includes proeutectic aluminum phase dispersed within the eutectic matrix. The braze alloy further comprises between about 0.07 and 0.25 percent magnesium that vaporizes during brazing.
The clad assembly is heated to a temperature effective to melt the low melting braze alloy without harming the core alloy. The braze melt is drawn by capillary action into the seams and, upon cooling, solidifies to form the desired braze bond. The result is a leak-free joint between components.
Brazing is particularly sensitive to the presence of oxygen and, therefore, is carried out in a vacuum. To further reduce oxidation, as the assembly is heated, but prior to braze melting, magnesium vaporizes and scavenges residual oxygen from the environment. Nevertheless, there remains alumina film on the surface of the cladding as the result of exposure to air during forming and assembly. This film may be diminished by the magnesium vaporization. As the brazing cycle progresses, the molten alloy swells and exudes through thin regions, discontinuities, or other defects in the film, thereby breaking up the film and dispersing its material in the braze melt. Failure to disrupt the surface oxide film inhibits uniform flow of braze alloy into the seam and may produce a defect in the brazed joint. Thus, the formation of defects in the oxide film and the exudation of molten alloy to break up the film are critical steps in the braze bonding process.
Therefore, it is an object of this invention to provide an improved method for brazing a clad aluminum sheet assembly into an integral structure that facilitates formation of leak-free joints between components.
More particularly, it is an object of this invention to provide an improved vacuum brazing heating cycle for bonding clad aluminum sheet alloy components, which cycle includes a dwell wherein the braze alloy is maintained in a partially molten state not only to enhance magnesium vaporization and gettering of oxygen from about the assembly, but also to promote exudation of the molten braze alloy to disrupt and disperse residual surface oxide, thereby providing more uniform flow of braze alloy into seams between components to produce leak-free joints in the bonded product.